1. Field of the Invention
This invention relates to an exhaust gas purification system of an internal combustion engine for removing an unburnt HC component from the exhaust gas.
2. Description of the Related Art
Internal combustion engines are ordinarily provided with a catalyst (a three-way catalytic converter) in the exhaust system which removes HC, NOx and CO components in the exhaust gas generated by the engine. However, when the engine is cold started, for example, and the catalyst is not activated, an unburnt HC component is immediately exhausted to the atmosphere.
For that reason, there is proposed an exhaust gas purification system in Japanese Laid-Open Patent Application Hei 4(1992)-17,710. In the system, the exhaust pipe 5 is branched out at a location downstream of the catalysts 6, 7. The bypass 9 is provided with an adsorber 10 made of active carbon or activated charcoal and carried on two beds. The bypass is again merged to the exhaust pipe 5 downstream of the adsorber. An Exhaust Gas Recirculation (EGR) conduit 15 is connected before the merging point where the bypass 9 is located such that an unburnt HC component, when adsorbed, is recirculated to the engine intake system through the EGR conduit 15. A valve 14 is provided in the bypass 9 downstream thereof to open or close the bypass 9. Moreover, another valve 13 is provided in the exhaust pipe 5 to close the same so as to lead the exhaust gas to flow into the bypass 9. Japanese Laid-Open Patent Applications Hei 5 (1993)-171,929 and Hei 6 (1994)-101,461 disclose similar systems.
In the prior art system (described in 4-17,710), if the bypass valve 14 sticks in the closed position when the exhaust pipe valve 13 closes the exhaust pipe 5, the exhaust system becomes clogged thus rendering the engine to stall. Moreover, since the bypass 9 is connected with the EGR conduit 15 downstream of the adsorber 10, the bypass valve 14 is indispensable and can not be omitted. Therefore, the system requires that both the exhaust pipe valve 13 and the bypass valve 14 are provided separately and independently of each other. In the prior art system, without the bypass valve 14, the unburnt HC component adsorbed will not flow to the EGR conduit 15, but flow to the exhaust pipe 5 and to the atmosphere.
The bypass valve 14 and the exhaust pipe valve 13 is subjected to the hot exhaust gas and should accordingly have the property of being heatproof. The provision of such a kind of valve is highly disadvantageous in terms not only of manufacturing cost but also of durability and reliability of the exhaust system. Furthermore, as mentioned above, the separate provision of the valves will increase the probability that the exhaust pipe becomes clogged.
Furthermore, since the bypass is connected with the EGR conduit 15 downstream of the adsorber 10, the EGR conduit may be long, depending on the engine layout. In an ordinary vehicle, the engine is usually mounted at its front and the engine exhaust system including a catalyst extends toward the rear of the vehicle under the floor structure. The distance between the connection point of the bypass with the EGR conduit and the engine increases as the connection point is situated closer to the rear of the vehicle.
Similar disadvantages are found in the other prior art systems (disclosed in 5-171,929 and 6-101,461).